Resistance compound



March 22, 1938. s BLQQMENTHAL 2,111,742

RES I STANCE COMPOUND Filed Sept. 9, 1953 7 l ll 1ll Wfifmnmuv 1 Patented Mar. 22, 1938 signor to Corporation of America, a

corporation of Delaware Application September 9, 1933, Serial No. 688,745

Claims.

My invention relates to moldable compounds and to methods of treating articles molded therefrom. More specifically stated, my invention, though not limited thereto, particularly pera tains to resistors of the types utilized in radioreceivers and the like.

According to the teachings of the prior art, many methods have been proposed for making resistor devices of the high resistance type. Among such methods may be mentioned the application of India ink or a resistance-materialcontaining paint to a Bakelite or hard rubber base to provide a resistance film over whichrides a movable contact element, and resistors have also been constructed wherein the resistance material itself has been distributed throughout a body of insulating material such as a phenol condensation product or the like.

Resistors of the first mentioned type do not stand up well in use, since a permanent bond cannot be established between the resistance film and the supporting material, even though the surface of the latter is roughened by sand-blasting or by an equivalent treatment.

It is, accordingly, an object of my invention to provide an improved moldable material to which, after molding, a matte surface may readily be imparted by chemical means.

Another object of my invention is to provide a novel method of manufacturing moldable material.

It is also a well known fact that, when using commercial resistors of the types referred to, there is an abrupt change in resistance as the movable element, contacting the resistance film, arrives adjacent to a terminal connection.

Another object of my invention, therefore, is to provide a resistor wherein the change in resistance is less abrupt than in devices heretofore known.

A still further object of my invention, ancillary to the object first above mentioned, is, to provide an improved moldable material and a method whereby said material may be given an ornamental surface subsequent to the molding operation.

Another object of my invention is to provide an improved insulating base for variable or fixed resistors to which coatings of resistance material or metals may be caused to firmly adhere.

In practicing my invention, I prefer/in brief, to utilize for resistor bases, knobs, panels and the like a compound including a material, (a) such as the phenol condensation product known as Bakelite, or other moldable material slightly soluble in a reagent such as sodium hydrate, and

(b) comminuted material which is unaffected by the said reagent. Other vehicles (a) for the comminuted material (1)) may also be used, such as the urea-formaldehyde condensation product known as Vinylite or the thiourea condensation product commercially called Plaskon, both of which are attacked by a solution of sodium hydrate. The compound (i. e., a+b) isfirst subjected to a suitable molding process which gives to it the desired configuration and the resulting object is next treated by the proper reagent to roughen the surface by dissolving some of the vehicle carrying the reagent-resisting material. The said object, if a resistor is being made, thereafter, is supplied with low-resistance terminal layers, is given a coating of high resistance material such as graphite, or the like, and baked.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing, in which Figural is a top plan view of a variable resistor base constructed according to my invention, and Figure 2 is a view in cross-section of the same base taken along a line corresponding to line 2-2 in Figure 1.

Referring to the drawing, illustrative of a potentiometer type resistor constructed according to my invention, the said device is constituted by a circular element 5 molded from my improved compound, which element is provided on one face thereof with a plurality of circular ridges 3 and 5. These ridges define a substantially annular shal low trough wherein the resistance material i itself is deposited, as will hereinafter more clearly be explained.

It will be noted that the resistor element is provided with a plurality of openings 9 and ii, for the purpose of accommodating conductorterminals (not shown) and a central opening it to accommodate an actuating shaft for a movable contact device (also not shown) Immediately adjacent to each of the openings t and ii,

" and in intimate contact with the base, as indicated by a plurality of shaded areas it and ii, is a deposit of conductive material such as copper, silver, or like, over which extends the resistor material l per se.

The conductor terminals (notshown) which; in the complete device, extend through the openings 9 and H, make contact, preferably, only with the outer surface of the resistance material and are not in contact with the low resistance metallic deposits. The said deposits, however, serve to so reduce the resistance of the end portions of the resistance layer that the change in resistance, as the movable contact device moves from the resistive layer 1 to come to rest upon that portion thereof immediately above one of the deposits is less abrupt than in similar devices constructed previous to my invention.

It should also be apparent, from the foregoing, that my invention is applicable to variable resistors of the usual type wherein the potentialdividing feature is not necessary. In such case, only one resistance-reducing metallic areaneed be applied to the base. Furthermore, it lies within the scope of. my invention to provide one or more areas having low conductivity spaced apart along the resistor base.

In order that my invention shall more fully be disclosed, the exact composition of my preferred resistor base material, and also the preferred method of preparation of the base and of application of the resistance layer thereto will now be explained.

The ingredients which I have found most suitable are molding resins known as GE #1358 and Bakelite 0220, which may be obtained from the General Electric Company and Bakelite Corporation, respectively, powdered glass which, after screening through a 200 mesh U. S. screen has the composition 63% by weight retained on a 325 mesh 11. S. screen and the remainder, 37 passing through the 325 mesh U. S. screen, and lubricating wax commercially known as Montan wax.

Assuming that a batch of 1000 grams of molding material is to be prepared, the following'proportions may be followed:

Grams Resin 350 Powdered glass 645 Montan wax 2.1

The glass powder. thev resin powder, and the wax in the form of shavings are mixed dry in a vertical paddle mixer, ball mill, or in any other suitable device well known to those skilled in the art. This mixture is then rolled between hot rolls, one of which is maintained, usually by steam, ,at a temperature of the order of 200 F. and the other at a temperature of the order of 250 F. The material is repassed through the rolls a number of times, in order to ensure homogeneity. Specifically, I have found that ten passes enable the desired consistency to be obtained.

After rolling, the material is allowed to cool and is re-ground to pass an mesh screen. It is then ready for the molding step.

In view of the fact that the 80 mesh material resulting from the first step of my process has a bulk factor of approximately 4:1, it is desirable that the resistor bases be partially preformed before the final molding step. If the bases are to be circular in configuration, obviously the next step in the process is to cold-mold from the material a number of circular discs.

The discs are next subjected to a final molding process, in a suitable steel mold, using heat and pressure, the temperature, however, and the duration of heat and pressure being such that the resin is not completely polymerized. They are A-W8X) and leaves a roughened surface constituted by projecting portions of innumerable glass particles. The reagent bath for the specific moldable composition hereinbefore referred to may be constituted by a solution of caustic soda, caustic potash, acid, or the like.

Preferably, the bath is 10% by weight ofanhydrous NaOH and by weight of water and is maintained at a temperature of C. About cc. of solution is allowed per resistor and a large number can be etched at one time.

As a general rule, the etching process requires 20 seconds, after which the bases are removed and the etching bath adhering to them is neutralized by dilute hydrochloric acid after which they are carefully washed in boiled distilled water, dried, and baked for 24 hours at a temperature of 100 C. to complete the polymerization of the resin.

The next step in my process is the provision of one or more metallic areas such as the areas 15 and I7 shown in Figure 1-of the drawing. The metal may be applied by the Schoop spraying process or by electro-plating, and it is also obvious to one skilled in the art that it may be applied by a precipitation process, the roughened surface causing the metal to firmly adhere to the base.

After the requisite number of terminal areas have been applied, they are next inter-connected by an overlying coating of resistance material. For this purpose, I prefer to utilize a paint containing one part by weight of solids, the solids consisting of 65% powdered resin, such as Bakelite, 17.5% of carbon black, 17.5% of graphite, and four parts by weight of a liquid vehicle, all of the percentages being by weight. Naturally, however, since the various kinds of carbon black tend to raise the resistance, the numerous varieties of graphite tend to lower the resistance and the percentages and sources of the several elements determine the resistance of a single coating, the proportions used are governed by the required resistor characteristics and a large number of samples need not be given.

The liquid consists of 50% by weight of acetone, the resin solvent, and 50% by weight of amylacetate, the thinner, with the proportions varied to give the proper drying time. For slow drying, the amount of amylacetate is increased,

and vice versa for quick drying.

It will be noted that the ridges shown in the drawing provide a liquid-retaining channel. The coating step of the process, therefore, consists in depositing in this channel a definite volume of the resistance material paint. After a short period of drying at room temperature, the base is baked at 100 C. for 10 minutes, followed by a bake at C. for 30 minutes. The capillarity of the surface provided by the glassparticles and the polymerization of the resin in the paint during the baking process, cause the resistance film to bond so firmly to the base that it can only be removed with difiiculty and no definite plane of demarcation is apparent.

The application of the conducting film to the etched base is, perhaps, the most critical step in my process. This step is preferably performed in a clean atmosphere having a definite moisture content and which is maintained at a definite temperature. Careful control at this point eliminates the variations in baking time which have been encountered if proper precautions are not taken. Such variations in baking time obviously give rise to resistors which vary in conductivity and increase the proportion of reiects".

From the foregoing, it might inferred that my invention is limited to the production of resistors of non-taper types. Such is not the case, however. Variation in resistance along the channel may be secured through the application of successive coats of resistance material paint, or by the application at the same time to different sectors of the channel of accurately measured volumes of difierent paints (whose total volume is about .1 cubic centimeter) which, when baked, differ in conductivity, but which, while slightly wet, merge at their junctions through capillarity to provide smooth variation in resistance, as well as a smooth physical junction.

Another method of obtaining taper, though not the preferred method at this time, is to deposit a tapered metallic layer within the channel before the resistance material paint is applied thereto.

My invention, particularly when applied to resistor bases,. gives rise to many' advantages, among which may be mentioned. the following:

1. The bases are extremely rugged in construction and resist abrasive wear from the contact member, since'the fllm is bonded firmly thereto.

2. Low terminal resistance is produced because a short-'circuiting layer of material can be deposited upon the etched base, with full asurance that it will-remain firmly adherent. This is impossible when a polished base is used.

3. Smooth joints may be produced between adjacent sections of a tapered resistor because of the capillarity of the etched base.

chanical abrasive methods.

,6. By reason of the presence of the minute surface irregularities, a slight abrasive action is exerted upon the contact member which ceases as soon as the irregularities become glossed over. This results in a resistance film which is substantially impervious to wear. At the same time, the minute protuberances protect the resistance film against abrasion which has amply been proved by test during which at least 100,000 rubs from one end to another of a resistor element by a contact element-were made, and during which test heavy pressure was maintained between the resistor base and the contact member.

Many other variations of'my process and vs;- riations in the form of the final resistor will at once be apparent to those skilled in the art to It will also be apparent that my moldable material and process which my invention pertains.

may be utilized for many other objects such as panels, cabinets, knobs, etc., the size, colors, and

shapes of the pieces of glass or other reagent resisting material being preferably so chosen as I claim as my invention: 1. The steps in a method of manufacturing resistors and the like which comprise providing an insulating base having an etchable surface,

etching said surface, depositing on the etchedsurface a resistance material paint including as one of its ingredients a polymerizable equivalent of the material removed by the etching process and thereafter baking the painted base, whereby the paint is hardened by polymerization and is homogeneously bonded to the base and a definite plane of demarcation is obviated.

2. The steps in a process of manufacturing a of time to partially polymerize the resinfand thereafter subjecting the article to a reagent .which removes the surface resin and wax to expose the comminuted material.

3. The process of manufacturing a resistanceunit which comprises intimately mixing a polymerizable resin, a comminuted material that resists reagents which attack the resin after polymerization and a wax to form a homogeneous mass, comminuting the mass, cold-molding a portion of the mass into approximately thedesired shape, subjecting the molded article to pressure and elevated temperature for a sufficient length of time to partially polymerize the resin, subjecting the articie.'to a reagent which removes the surface resin and wax to expose the comminuted material, depositing on an etched surface a resistance material paint including as one of its ingredients a polymerizable resin and thereafter baking the base to cause complete polymerization of the partially polymerized resin and to cause the paint to firmly adhere thereto by polymerization of the resin content thereof.

4. The steps in a process of manufacturing a resistance-unit which comprise intimately mixing a polymerizable resin and a comminuted vitreous material that resists reagents which attack the resin after polymerization, forming a portion of the mix, under pressure, into a resistanlce-unit-base while simultaneously subjecting the said portion to anelevated temperature sufficient to partially polymerize the resin, subjecting the formed base to an etching reagent which removes the surface resin to expose the comminuted vitreous material, depositing on an etched surface a resistance material paint including as one of its ingredients a polymerizable resin and thereafter baking the base to cause complete polymerization of the partially polymerized resin and to cause the paint to firmly adhere thereto by polymerization of the resin content thereof.

5. A resistance unit including a base constituted by amass of polymerized resin from a surface ofwhich projects a plurality of small vitreous particles and over which surface extends a layer of resistance material and polymerized resin, there being no definite line of demarcation between the surface layer and the base. 

